LOL. well it kinda depends on exactly what you have been doing (constant intense arm/upper body strength workouts?) and how much extra padding you had in the arm area that may have been covering it up.
In other words you might be seeing a combination of an increase in muscle and decrease in fat (or just an increase in muscle under a not very thick layer of fat/skin). Or, if you have recently done a really hard workout that you were not used to, there could be a little inflammation of the muscles and surrounding tissue while they repair and rebuild (you would be quite sore if this were the case).
it's also possible that you are more aware of and focussed on your body and may have just noticed what was always there!
35kg lost. (November 2005 - October 2006)
15kg gained again (as at October 2010).
Back to the drawing board - Let's do this thing!
"You can't change the winds, but you can change the sails"
"Reach out and take control of what lands in your lap"